Hydrogen sulfide is a contaminant that is frequently encountered in industrial gas streams of commercial importance. Examples include many sources of natural gas, refinery gases produced from hydrodesulfurization processes, and most coal-derived gas streams. The hydrogen sulfide must generally be reduced to a rather low level, frequently 4 parts per million (ppm) or less, before the gas stream may be used as a fuel or a synthesis gas or be discharged to the atmosphere. Processes for removing hydrogen sulfide from gas streams (called sweetening) are generally both capital- and energy-intensive.